There's nothing worse than to discover that someone has Johnnie counted three or four times under the same type of document, and it isn't even the same person.
But in the days where an actual Census Taker visited homes to take the information, duplications occurred. Grandchildren visited their grandparents, cousins hung out to play, and nieces and nephews were being babysat by Auntie Louise. It just happened.
The one nice thing about finding a duplicate is that if you were questioning a connection to a potential grandparent or aunt/uncle, this might help clarify that situation.